A quadrature amplitude modulation, referred to hereinafter as QAM, has been increasingly employed in a multi-channel digital communication system due to its high efficiency capability. Concept of a system configuration of a typical QAM communication system is schematically illustrated in FIG. 1. Explanation will be given hereinafter representatively with the I channel because the circuit configurations of I and Q channels are symmetrical with each other. In a transmitting station, an internal frequency band (referred to hereinafter as IF) or a base band (both referred hereinafter to representatively as IF) of an I channel and a Q channel are input to input terminals of first digital roll-off filters 102ai and 102aq, respectively. Circuit configuration of roll-off filters will be described later in detail. Frequency characteristics of the first roll-off filters are determined by the tap rating ratios stored in a first read-only memory, referred to hereinafter as a ROM, 113.
Output of first roll-off filter 102ai is input to a digital-to-analog (referred to hereinafter as D/A) converter 131i. Unnecessary higher frequency spectrum generated in the output of D/A converter 131i is eliminated by a low-pass filter 132i. Outputs from low-pass filters 132i and 132q respectively of the I channel and Q channel are input to a QAM modulator 134, to which an carrier frequency signal is also input from an carrier generator 133. A QAM modulated radio frequency signal is transmitted via a radio frequency amplifier (not shown in the figure) to a receiving station.
In the receiving station, a preamplifier (not shown in the figure) amplifies and converts the received radio frequency signal to an IF signal, which is then input to a QAM demodulator 154, to which a local frequency signal is input from a local frequency oscillator 156. I channel and Q channel signals output from QAM demodulator 154 is input via a low-pass filter 152i to an analog-to-digital (referred to hereinafter as A/D) converter 151l. Each bit line of a parallel digital signal output from A/D converter 151i is input to a second roll-off filter 102bi. Frequency transmission characteristic of second roll-off filter 102bi is determined by tap rating ratios stored in, and output from, a ROM 6-1.
Frequency characteristics of the first and second roll-off filters are chosen such that overall transmission characteristics, i.e. a total of the frequency characteristics of the two filters in each channel, allow the second roll-off filter to output signal pulses in an adequately good shape which causes no intermodulation in the QAM-modulated signals, as well as to eliminate unnecessary upper frequency spectrum generated from the circuits, such as D/A converter, etc.
Even though the overall frequency characteristics that are the sum of both the first and second roll-off filters are set so that the pulse form at the output at second roll-off filter is in a good shape, fading or some other factors in the transmission system always varies the transmission characteristics, such as frequency vs amplitude, or frequency vs phase-delay, which accordingly deteriorate the pulse forms output from the second roll-off filter.
In order to remedy this deterioration an automatic equalizer 160i is provided at the output of the second roll-off filter 102bi. Output of second roll-off filter 102bi is input via a pulse divider 146i, which returns the signal to have the symbol rate, to a first input terminal of automatic equalizer 160i. Moreover, an output of roll-off filter 102bq of Q channel is input to a second input terminal of automatic equalizer 160i of the I channel. Symmetrically the same cross-connection is done in the Q channel.
Automatic equalizers are formed of roll-off filter, the frequency characteristics of which are variably determined by tap rating ratios given from a calculating circuit 7. Calculating circuit 7 is formed of a micro computer system which monitors the pulse formed of the signal output from the automatic equalizer 160i and calculates optimum values of the factors so that pulse form of the signal output therefrom is satisfactorily in a good shape.
The output signal is also input to an carrier regeneration circuit 155, the output of which is fed back to control local oscillator 156.
A problem of this circuit configuration is that the provision of the automatic equalizer causes a cost increase in manufacturing the receiving station.
A more important problem is that, when the frequency characteristics are deteriorated by some elements located after the first roll-off filter and the element which carried deteriorations must be urgently removed, it is impossible to locate that element by checking eye patterns of the waveforms at check points A1, A2, A3, B3, B2, B1 and B0, each located after the first roll-off filter, without time-consuming manual operations. This is because, even in a normal state where the second roll-off filter is outputting satisfactory waveforms the waveforms in the stages between the two roll-off filters are not in a good shape viewed in the eye diagram.